CHICAGO, Sept. 18 (Xinhua) -- Using a state-of-the-art climate model to simulate the extreme warming of the Early Eocene Period, which is considered an analog for Earth's future climate, researchers at the University of Michigan (UM) found that the rate of warming increased dramatically as carbon dioxide levels rose.
The researchers determined that the large increase in climate sensitivity they observed, which had not been seen in previous attempts to simulate the Early Eocene using similar amounts of carbon dioxide, is likely due to an improved representation of cloud processes in the climate model they used, the Community Earth System Model version 1.2, or CESM1.2.
In simulations of the Early Eocene, the researchers found a reduction in cloud coverage and opacity that amplified CO2-induced warming.
The same cloud processes responsible for increased climate sensitivity in the Eocene simulations are active today, according to the researchers.
"Our findings highlight the role of small-scale cloud processes in determining large-scale climate changes and suggest a potential increase in climate sensitivity with future warming," said UM paleoclimate researcher Christopher Poulsen, a co-author of the study.
"The sensitivity we're inferring for the Eocene is indeed very high, though it's unlikely that climate sensitivity will reach Eocene levels in our lifetimes," said Jessica Tierney of the University of Arizona, the study's third author.
The finding has far-reaching implications for Earth's future climate.
The Early Eocene, roughly 48 million to 56 million years ago, was the warmest period of the past 66 million years. It began with the Paleocene-Eocene Thermal Maximum, which is known as the PETM, the most severe of several short, intensely warm events.
The Early Eocene was a time of elevated atmospheric carbon dioxide concentrations and surface temperatures at least 14 degrees Celsius (25 degrees Fahrenheit) warmer, on average, than today. Also, the difference between temperatures at the equator and the poles was much smaller.
Geological evidence suggests that atmospheric carbon dioxide levels reached 1,000 parts per million in the Early Eocene, more than twice the present-day level of 412 ppm. If nothing is done to limit carbon emissions from the burning of fossil fuels, CO2 levels could once again reach 1,000 ppm by the year 2100, according to climate scientists.
The study is published on Wednesday in the journal Science Advances.